Many food products are required to be packaged in glass or plastic containers under a partial vacuum to prevent spoilage or to preserve flavor. It is important that a closure for such a container be able to properly seal the neck finish of the container to maintain the vacuum in the container until it is opened for consumption. It is also important to the packager to be able to visually inspect the container prior to shipment to ascertain whether the product contained in the container is still under sufficient amount of negative pressure.
Various closures have been proposed for indicating the presence or absence of a negative pressure in the container. For example, U.S. Pat. No. 2,194,004 discloses a metal closure used with hot filled products and indicates to the packager when the closure has been properly sealed and when all of the air has been removed. This is indicated by the closure being snapped from an upwardly-bowed position to a downwardly-bowed position.
U.S. Pat. Nos. 4,616,761 and 4,678,082 disclose thermoplastic closures which have central portions that are deflected to indicate the presence or absence of vacuum in the container to which the closure is sealed. This type of closure requires a significant amount of vacuum before the center button of the closure moves to a vacuum-indicating position.
However, the deflectable portion of the closure has only two positions and the portion which connects the deflectable portion to the periphery of the container is very small. Moreover, the connecting portion must also be deflected between two positions.